One possible aid to an arbitrary selection of a design flood in the absence of economic considerations is to consider the probability that the structure will have its capacity exceeded one or more times during its design life. This probability is called the ¿risk of failure¿ in the UK Flood Studies Report ¿ Volume 1 (Natural Environment Research Council, 1975).

There are some situations in which the structure for which the design flood is to be determined has a clearly defined finite life. One example is a temporary cofferdam used during the construction of a permanent dam as in the first worked example on the economic selection of a design flood. However, in other situations such as permanent dam which is not to be removed at the end of a specific period, the length of time during which it is subject to the risk of having its design capacity exceeded by floods is very long and the probability of this happening becomes very high. It should be noted that in these circumstances this length of time or ¿structural design life¿ can be expected to be much more than the ¿economic design life¿. This problem of determining the design life of a project is complicated further by the presence of climate change and the potential for ¿non-stationarity¿ of the potential for occurrence of flood flows.